1. Field of the Invention
The present invention relates to a wide-angle zoom lens system, for single lens reflex cameras, which has an angle-of-view (=2.omega.) of more than 90.degree. at the short focal length extremity.
2. Description of the Related Art
In a single lens reflex camera, it is necessary to maintain a long back focal distance because a quick-return mirror is positioned behind the photographing lens system. For this reason, a retrofocus lens system having a negative lens element and a positive lens element, in this order from is the object, is generally employed for a wide-angle lens system. However, in a retrofocus lens system, it is extremely difficult to correct distortion and field curvature in the vicinity of the short focal length extremity while securing a long back focal distance and maintaining an overall length of the lens system shorter. For example, in a wide-angle zoom lens system in which a diaphragm S is positioned between the negative front lens group and the positive rear lens group, if, an attempt is made to effectively correct distortion and field curvature, there is only one of the following two ways:
(i) positioning the positive lens element in the negative front lens group more distant from the diaphragm S; and positioning the negative lens element in the positive rear lens group more distant from the diaphragm S; or PA1 (ii) providing the positive lens element and the negative lens element with stronger power.
However, in the former, the diameter of the front-most lens element in the front lens group has to be increased, and also the diameter of the rear-most lens element in the rear lens group has to be increased, and furthermore the entire length of the lens system has to be increased. On the other hand, in the latter, if the power of the positive lens element in the negative front lens group is increased, the power of other negative lens elements in the negative front lens group has to be increased at the same time. This causes an increase of aberration of higher-order. AS a result, the rate of change in distortion and filed curvature with respect to the change in the angle-of-view increases, in particular, in a range where the angle-of-view is larger; and coma of higher-order remains, so that maintaining optimum optical performance becomes difficult.
Due to the above-mentioned problems, in a zoom lens system, particularly, in a wide-angle zoom lens system in which the angle-of-view at the short focal length extremity is more than 90.degree. for the purpose of miniaturization and in order to correct aberrations other than distortion and field curvature, these distortion and field curvature have to be allowed to remain to some extent.